Techniques for the separation of a solute from a solution of the solute and a relatively non-volatile solvent are often so expensive and energy consuming that their application, on a large scale, is limited to the production of substances whose economic, political or social value transcends the criteria of the marketplace. For example, the desalinization of sea water for the production of potable water or crop irrigation water is at present practicable only in a very few arid regions. Different approaches including distillation, electrodialysis based on ion exchange, reverse osmosis using hydrostatic pressure, or vacuum freezing cannot as yet provide a return that is generally commensurate with the required large investment of capital and energy. Material problems, such as the difficulty of fabricating durable membranes of the required separating capacity, have not yet been satisfactorily solved. In desalinization, efficiency of the various technologies is adversely affected by the need to protect machinery against corrosion and scaling and to absorb or dispose of process heat before it is added to the environment. Similar considerations impeded, in many cases, development of large scale purification, separation and concentration techniques for solutions of diverse composition.
One apparatus and method for separating a solvent from a solute is disclosed in our co-pending application Ser. No. 851,918, METHOD AND APPARATUS FOR SEPARATING SOLVENT FROM SOLUTE. This application discloses a process whereby a volatile solvent is separated from a solution containing the solvent and a non-volatile solute by establishing a temperature gradient across a solvent-absorbing and gas-entraining matrix material infiltrated with the solution. The process results in concentration of the solution near the hotter matrix surface and dilution of the solution near the cooler matrix surface. In a specific application, salt water, such as sea water, is desalinated across a matrix in which air is entrained. The salt water vaporizes somewhat and diffuses through microgaps of matrix-entrained air, the vapor then condensing near the cooler surface. This process works well under static conditions, and produces potable water from a 10.degree. C. surface and six times concentrated sea water from a heated (60.degree. C.) surface. However, it is now known that during continuous operation the condensing vapor absorbs the entrained air thereby requiring aeration or substitution of matrix material.
The present invention discloses an apparatus for separating a solvent from a solute at a relatively low temperature but which does not require a matrix material and which is continuous in operation without need for aeration or substitution of materials. The apparatus includes opposed, extended, relatively warm and cool surfaces spaced-apart so that a continuous vapor gap is present between the surfaces, the vapor gap being the functional equivalent of the microgaps of entrained air and/or vapor in the matrix material of our prior device described above. The solution (sea water) is disposed over the warmed surface, the volatile solvent (pure water) vaporizing somewhat as it is warmed and condenses on the cooled surface, the condensate being removed as potable water. The width of the gap is determined by the following two considerations: (1) the gap should be as small as feasible in order to minimize the diffusion distance of the solvent vapor from the warmed surface to the cooled surface, thereby permitting use of a relatively low temperature differential which can be in the range of 5.degree. C.-80.degree. C.; and (2) the gap must not be so narrow as to allow the solution to bridge the gap and contact the solvent condensate forming on the cooled surface.
Thus the present invention provides an apparatus for isolating a volatile solvent from a solution of the solvent and a relatively non-volatile solute, the apparatus including opposed, slightly spaced apart cool and warm extended surfaces, means for passing the solution over the warm surface, and means for recovering solvent condensate formed on the cool surface. In one embodiment of the invention, the cool surface consists of one side of a heat-conducting plate, the other side partially forming a chamber through which coolant flows. In a similar manner, the warm surface consists of one side of a heat-conducting plate, the other side partially forming a chamber through which a warming medium flows. Thus a temperature differential is continuously maintained across the entire extended surface areas.
In a particular embodiment, in order to facilitate a uniform flow of water over the respective surfaces, a thin fibrous, capillary material is disposed across each surface. In such case, the gap referred to above is that obtained between opposed layers of fibrous material. Means are provided for removal of the concentrate and for collection of the condensate. Specific embodiments of the invention include use of the apparatus aboard an ocean-going vessel whereby sea water utilized for engine cooling is also used for heating the warm surface, and ambient sea water is used for cooling the cool surface. This ordinarily wasted engine heat can thereby be utilized to provide the vessel's fresh water requirements. A further embodiment of the invention includes use of the apparatus as a combined heat exchanger-solute solvent separator in lieu of the conventional heat exchanger for condensing the steam discharged from the turbine turning an electric generator. Another embodiment of the invention discloses an apparatus for heating the warm surface by the rays of the sun and cooling the cool surface by heat radiation.
Although the specific embodiments relate to desalinization apparatus, it will be appreciated that the invention relates to the separation of any volatile solvent from a solution of the volatile solvent and a non-volatile solute. In this regard, it may be that the concentrate is of interest rather than the distillate. For example, the apparatus could be utilized to obtain maple syrup from maple sap. It will also be appreciated that both solvent and solute can be liquid as in the case of an alcohol the more volatile (solvent)--water (the less volatile solute) solution.